Certain engine configurations, in particular in-line four cylinder engines, are known to generate second order vibration unbalance. To offset this unbalance, a balancer including a pair of balance shafts is integrated into the engine. The balance shafts are carried in a housing below the crankshaft and rotate in opposite directions at twice the crankshaft speed. A crankshaft gear or chain drives the first balance shaft, which in turn drives the second balance shaft through a pair of drive and timing gears.
A common concern of balancers is gear rattle noise, accentuated at engine idle when the crankshaft rotational velocity fluctuates between firing pulses of succeeding cylinders. The velocity fluctuation causes the meshing teeth of the balance shaft drive and timing gears to lose contact and impact adjacent teeth.
Gear rattle may be reduced by including drag-inducing elements to increase the drag torque on the second balance shaft, thereby reducing the speed differential between the first and second balance shafts. Drag-inducing elements, such as oil pumps, driving chains, and journal bearings, may reduce rattle but at the expense of fuel economy and system efficiency.
In addition, balance shaft gears generally only have a pair of teeth in contact at a given time so these contacting teeth must be able to withstand the impact of the one upon the other. Therefore the gears are usually steel that may be carburized, quenched, and tempered after the final machining of the teeth to provide sufficient strength. Such heat treatment has the potential for distorting the dimensions of the teeth.